δ44/40Ca variability in shallow water carbonates and the impact of submarine groundwater discharge on Ca-cycling in marine environments

Shallow water carbonates from Florida Bay, the Florida Reef Tract, and a Mexican Caribbean fringing reef at Punta Maroma were studied to determine the range of Ca-isotope variation among a cohort of modern carbonate producers and to look for local-scale Ca-cycling effects. The total range of Ca-isotope fractionation is 0.4 parts per thousand at Punta Maroma, yielding an allochem-weighted average delta Ca-44/40 value of -1.12 parts per thousand consistent with bulk sediment from the lagoon with a value of -1.09 parts per thousand. These values are virtually identical to bulk carbonate sediments from the Florida Reef Tract (-1.11 parts per thousand) and from one location in Florida Bay (-1.09 parts per thousand) near a tidal inlet in the Florida Keys. No evidence was found for the similar to 0.6 parts per thousand fractionation between calcite and aragonite which has been observed in laboratory precipitation experiments. Combining these results with carbonate production modes and delta Ca-44/40 values for pelagic carbonates taken from the literature, we calculate a weighted average value of -1.12 +/- 0.11 parts per thousand (2 sigma) for the global-scale Ca-output flux into carbonate sediments. The delta Ca-44/40 value of the input Ca-flux from rivers and hydrothermal fluids is -1.01 +/- 0.04 parts per thousand (2 sigma(mean)), calculated from literature data that have been corrected for inter-laboratory bias. Assuming that the ocean Ca cycle is in steady state, we calculate a delta Ca-44/40 value of -1.23 +/- 0.23 parts per thousand (2 sigma) for submarine groundwater discharge (SGD) on a global scale. The SGD Ca-flux rivals river flows and mid-ocean ridge hydrothermal vent inputs as a source of Ca to the oceans. It has the potential to differ significantly in its isotopic value from these traditional Ca-inputs in the geological past, and to cause small changes in the delta Ca-44/40 value of oceans through time. In the innermost water circulation restricted region of northeastern Florida Bay, sediments and waters exhibit a 0.7 parts per thousand gradient in delta Ca-44/40 values decreasing towards the Florida Everglades. This lowering of delta Ca-44/40 is predominantly caused by local-scale Ca-inputs from SGD, which has a high Ca concentration (450 mg/L) and low delta Ca-44/40 value (-0.96 parts per thousand). Mixing calculations show that Ca inputs from SGD and surface water runoff from the Florida Everglades contribute between 8% and 60% of the dissolved Ca to the studied waters with salinities between 30 and 14, respectively. Similar degrees of circulation restriction between epeiric seas and oceans in the geological past may have also led to overprinting of sedimentary carbonate delta Ca-44/40 values in nearshore regions of epeiric seas due to local-scale cycling of seawater through coastal carbonate aquifers. Local Ca-cycling effects may explain some of the scatter in delta Ca-44/40 values present in the Ca-isotope evolution curve of Phanerozoic oceans. (C) 2011 Elsevier Ltd. All rights reserved.